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Ask the Applications Engineer—12: Grounding (Again)

Q. I’ve read your data sheets and application notes and also attended your seminars, but I’m still confused about how to deal with analog (AGND) and digital (DGND) ground pins on an ADC. Your data sheets usually say to tie the analog and digital grounds together at the device, but I don’t want the ADC to become my system’s star ground point. What do I do?

A. First of all, don’t feel bad that you are confused about what to do with your analog and digital grounds. So are lots of folks! Much of the confusion comes from the labeling of the ADC ground pins in the first place. The pin names, AGND and DGND, refer to what’s going on inside the component itself and do not necessarily imply what you should do with them externally. Let me explain.

Inside an IC that has both analog and digital circuits, such as an ADC, the grounds are usually kept separate to avoid coupling digital signals into the analog circuits. The diagram shows a simple model of an ADC. There is really nothing the IC designer can do about the wirebond inductance and resistance associated with connecting the pads on the chip to the package pins. The rapidly changing digital currents produce a voltage at point B which will inevitably couple into point A of the analog circuits through the stray capacitance. It’s the IC designer’s job to make the chip work in spite of this. However, you can see that in order to prevent further coupling, the AGND and DGND pins should be joined together externally to the same low impedance ground plane with minimum lead lengths. Any extra external impedance in the DGND connection will cause more digital noise to be developed at point B; it will, in turn, couple more digital noise into the analog circuit through the stray capacitance. Though an extremely simple model, this serves to illustrate the point.

Q. O.K., you’ve told me to join the AGND and DGND pins of the IC together to the same ground plane-but I am maintaining separate analog and digital ground planes in my system. I want them tied together only at one point: the common point where the power supply returns are all joined together and connected to chassis ground. Now what do I do?

A. If you have only one data converter in your system, you could actually do what the data sheet says and tie your analog and digital ground systems together at the converter. Your system star ground point is now at the data converter. But this may be extremely undesirable, unless you initially planned your system with this thought in mind. If you have several data converters located on different PCBs, the concept breaks down, because the analog and digital ground systems are joined at each converter on a number of PCBs. This is a perfect invitation for ground loops!

Q. I think I’ve figured it out! If I must join the AGND and DGND pins together at the device, and I want to maintain separate system analog and digital grounds, I tie both AGND and DGND to either the analog ground plane or the digital ground plane on the PCB, but not both. Right? Now, which one should it be, since the ADC is both an analog and a digital device?

A. Correct! Now, if you connect the AGND and DGND pins both to the digital ground plane, your analog input signal is going to have digital noise summed with it, because it is probably single-ended and referenced to the analog ground plane.

Q. So the right answer is to connect both AGND and DGND pins to the analog ground plane? But doesn’t this inject digital noise on my nice quiet analog ground plane? And isn’t the noise margin of the output logic degraded because it now referenced to the analog ground plane, and all the other logic is referenced to the digital ground plane? I plan to run the ADC outputs to a backplane tristate data bus which is going to be pretty noisy to begin with so I think I need all the noise margin I can get.

A. Well, nobody ever said life was easy or fair! You have reached the right conclusion by traveling a rocky road, but the problems you suggest-digital noise on your analog ground plane and reduced noise margin on your ADC outputs-really aren’t as bad as they seem; they can be overcome. It is clearly better to let a few hundred millivolts corrupt the digital interface than to apply the same corrupting signal to the analog input where the least-significant-bit for a 16-bit, 10-V-input-range ADC is only 150 µV! First of all, the digital ground currents on DGND pins can’t really be that bad, or they would have degraded the internal analog parts of the ADC in the first place! If you bypass the power pins of the ADC to the analog ground plane, using a good-quality high-frequency ceramic capacitor for high frequency noise (say 0.1 µF), you will isolate these currents to a very small region around the IC, and they will have minimal effect on the rest of your system.

You will incur some reduction in digital noise margin, but it is usually acceptable with TTL or CMOS logic if it’s less than a few hundred millivolts or so. If your ADC has single-ended ECL outputs, you may want to put a push-pull gate on each digital output-i.e., one with both true and complementary outputs. Tie the grounds of this gate package to the analog ground plane and connect the logic signals differentially across the interface. Use a differential line receiver at the other end which is grounded to the digital ground plane. The noise between the analog and digital ground planes is now common-mode-most of it will be rejected at the output of the differential line receiver. You could use the same technique with TTL or CMOS, but there is usually enough noise margin not to require differential transmission techniques.

However, one thing you said troubles me greatly. In general, it is unwise to connect the ADC outputs directly to a noisy data bus. The bus noise may couple back into the ADC analog input through the stray internal capacitance-which may range from 0.1 to 0.5 pF. It is much better to connect the ADC outputs directly to an intermediate buffer latch located close to the ADC. The buffer latch is grounded to your digital ground plane, so its output logic levels are now compatible with those of the rest of your system.

Q. I think I understand now, but why on earth didn’t you just call all the ground pins of your ADC AGND in the first place; then none of this would have come up in the first place?

A. Perhaps. But what if the incoming-inspection person connects an ohmmeter between these pins and finds out that they are not actually connected together inside the package? The whole lot will probably be rejected-and the IC may be blown! Furthermore, there is a tradition associated with ADC data sheets which says we must label the pins to indicate their true function, not what we would like them to be.

Q. O. K. Now, here comes a question I’ve been saving as your ultimate test! I have a colleague who designed a system with separate analog and digital ground systems. My colleague says that, with the ADC’s AGND pin connected to the analog ground plane and the DGND pin connected to the digital ground plane, the system is working fine! How do you explain this?

A. First of all, just because a practice is not recommended doesn’t necessarily mean you can’t get away with it some of the time and thereby be lulled into a false sense of security. (This is one of the lesser-known of Murphy’s Laws). Some ADCs are less sensitive to external noise between the AGND and DGND pins, and it may be that your colleague picked one of those by accident. There could be other explanations-which would require that we explore your colleague’s definition of “working fine”-but the point is that the ADC’s specifications are not guaranteed by the manufacturer under those operating conditions. With a complex component like an ADC, it is impossible to test the device under all possible operating circumstances, especially those which aren’t recommended in the first place! Your friend got lucky this time, but you can be sure that Murphy’s law will ultimately catch up with him (or her) if this practice is continued in future system designs.

Q. I think I understand the ADC grounding philosophy now, but what about DACs?

A. The same philosophy applies. The DAC’s AGND and DGND pins should be tied together and connected to the analog ground plane. If the DAC has no input latches, the registers driving the DAC should be referenced and grounded to the analog ground plane to prevent digital noise from coupling into the analog output.

Q. What about mixed-signal chips which contain ADCs, DACs, and DSPs such as your ADSP-21msp5O voiceband processor?

A. The same philosophy applies. You should never think of a complex mixed-signal chip, such as the ADSP-21msp50, as being only a digital chip! The same guidelines we’ve just been discussing should be applied. Even though the effective sampling rate of the 16-bit sigma-delta ADC and DAC is only 8 ksps, the converters operate at an oversampling frequency of 1 MHz. The device requires an external 13-MHz clock, and an internal 52- MHz processor clock is generated from it with a phase-locked loop. So you see, successful application of this device requires an understanding of design techniques for both precision- and high-speed circuits.

Q. What about the analog and digital power-supply requirements of these devices? Should I buy separate analog and digital power supplies or use the same supply?

A. This really depends on how much noise is on your digital supply. The ADSP-21msp50, for example, has separate pins for the +5- V analog supply and the +5-V digital supply. If you have a relatively quiet digital supply, you can probably get away with using it for the analog supply too. Be sure to properly decouple each supply pin at the device with a 0.1-µF ceramic capacitor. Remember to decouple to the analog ground plane, not the digital ground plane! You may also want to use ferrite beads for further isolation. The diagram below shows the proper arrangement. A much safer solution is to use a separate +5-V analog supply. You can generate the +5 V from a quiet +15-V or +12-V supply using a three-terminal regulator, if you can tolerate the extra power dissipation.

Author

Walt Kester

Walt Kester is a corporate staff applications engineer at Analog Devices. During his many years at ADI, he has designed, developed, and given applications support for high speed ADCs, DACs, SHAs, op amps, and analog multiplexers. An author of many papers and articles, he prepared and edited 11 major applications books for ADI’s global technical seminar series; topics include op amps, data conversion, power management, sensor signal conditioning, mixed-signal circuits, and practical analog design techniques. His latest book, Data Conversion Handbook (Newnes), is a nearly 1000-page comprehensive guide to data conversion. Walt has a B.S.E.E. from NC State University and an M.S.E.E. from Duke University.

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